The essential requirements for a ceramic material to be used as a thermally insulating component of an engine part, are good mechanical and thermal properties. For a ceramic material to be used for high temperature applications, such as a combustion environment in an engine, it should have high toughness. Toughness is defined as an improved resistance to fracture. Fracture may occur due to either inherent flaws in the ceramic microstructure or due to flaws sustained in the use of the ceramic at high temperatures. Fracture may also occur because of different thermal coefficient of expansion of the ceramic component and the metallic engine part. A ceramic material should also possess low thermal conductivity to enhance its insulating properties.
It is known that mullite (3Al.sub.2 O.sub.3 .multidot.2SiO.sub.2) is a good refractory material offering desirable properties at high temperatures such as low thermal expansion, low thermal conductivity, good hardness, good creep resistance, high melting point and good resistance to chemical attack. However, mullite, by itself, typically lacks the strength to be an engine component.
It is also known that zirconia (ZrO.sub.2) is a very good refractory material. The problem with zirconia, when used by itself for a thermally insulating component of an engine part, is that it de-stabilizes from tetragonal phase to monoclinic phase, losing up to 25% of its strength, when exposed to engine operating temperatures, moisture and the by-products of combustion. Several inventors have attempted to toughen the zirconia matrix by the addition of fibers and whiskers. U.S. Pat. No. 4,804,643 issued Feb. 14, 1989 to Chyung et. al discloses the use of silicon carbide (SiC) whiskers to reinforce the zirconia matrix to achieve a 25% gain in fracture toughness. However, to achieve this gain, the whisker-toughened zirconia is sintered by hot-pressing at high pressures in the range of 5-10 Kpsi. Not only is this process cost prohibitive for making a large number of parts, but the presence of SiC whiskers enhances the thermal conductivity of the material, which is an undesirable property for a thermally insulating component.
U.S. Pat. No. 4,774,209 issued Sep. 27, 1988 to Gadkaree et. al discloses the use of a combination of silicon carbide whiskers and zirconia to reinforce the mullite matrix. Gadkaree et. al also disclose a process of combining zirconia and mullite which includes first compounding a dry batch of the appropriate oxides, Al.sub.2 O.sub.3, ZrO.sub.2 and SiO.sub.2, and after the dry-mixing of these oxides, a liquid vehicle is added to form a slurry. It has been found in the present invention that the use of a ceramic material produced from such a process, for making an insulating component for an engine part, is not very beneficial because the material is susceptible to fracture due to inherent flaws in the grain microstructure as a result of the limitations inherent in mechanical dry-mixing of the oxides. Further, the presence of SiC whiskers is undesirable because they enhance the thermal conductivity of the thermally insulating component.
U.S. Pat. No. 4,657,877 issued Apr. 14, 1987 to Becher et. al discloses another mullite matrix with SiC whiskers and zirconia. This patent also discloses the dry-mixing of the mullite and zirconia particulates and SiC whiskers and subsequent sintering by hot-pressing. Further, Becher avoids the addition of a stabilizing aid such as yttria, primarily to avoid the reaction of SiC whiskers with cubic zirconium which would severely detract from the physical properties of the composite.
U.S. Pat. No. 4,519,359 issued on May 28, 1985 to Dworak et. al discloses a mullite matrix with zirconia or hafnia, and stabilizers such as magnesium oxide, calcium oxide, or yttrium oxide. Dworak further suggests that the production of the ceramic engine component is not tied to the use of a particular starting material. Contrary to this suggestion, it has been found that for application in an engine combustion atmosphere, the performance of the mullite-zirconia matrix is tied to the starting raw materials and process by which the starting raw materials, i.e., mullite and ZrO.sub.2, are mixed.
In the present invention, it has been found that if a mullite-zirconia matrix contains no sintering aids or stabilizers, and further if ZrO.sub.2 and mullite are chemically mixed in a molecular state as described by H. Shiga et. al, in "Sol Gel Synthesis and Sintering of Oxide Doped Mullite-ZrO.sub.2 Composite Powders", Ceramic Transactions, Vol. 22, 1991, rather than mechanically mixed in a powder form, the molecular level mixing of ZrO.sub.2 and mullite yields a very fine (0.1-0.5 microns) grain size upon sintering and improves the transformation toughening of tetragonal zirconia. It has further been found that there is a synergism between the starting raw materials and the sintering process, i.e., a thermally insulating component made from a chemically mixed and stabilizer free, mullite-ZrO.sub.2 mixture, can be slip-cast and pressureless sintered to yield a composite having a substantially net desired shape, and a significant enhancement in mechanical properties.
It is desirable to have a mullite-zirconia ceramic material that can be processed in a manner such that a grain size of less than 1 micron is attained. It is also desirable that a thermally insulating component for an engine part exposed to high temperature, high pressure, combustion gas environment in an engine, have a very low probability of failure. It is further desired that a thermally insulating ceramic component have high toughness and low thermal conductivity and be processed in a manner so that minimum machining of the component is required and high production rates can be achieved. The present invention is directed to overcome one or more of the problems as set forth above.